This invention relates to hypodermic syringes.
In the often high pressure conditions of medical work, the accidental puncture or scratch with a needle, commonly known as a needlestick injury, and consequent risk of infection with, for example, HIV or hepatitis is becoming increasingly hazardous to the profession and potentially more costly to health authorities.
Detailed statistics of needlestick injuries are not generally published in the UK but in the USA such information is more readily available. In 1994 there were some 800,000 cases reported of which 16,000 involved infection with HIV. It is estimated that in the USA, these injuries cost, excluding legal or insurance expenses, $1.8 billion pa.
The requirements for a syringe that will prevent needlestick injuries were analysed and listed as follows:
1. Reliability and ease of use.
2. Automatic and complete retraction of the needle after injection.
3. The capability to retract a needle bent intentionally or accidentally.
4. Re-exposure of the needle is impossible.
5. Suitable for production in larger sizes with an offset needle.
6. Supplied with needle fitted and sheathed.
7. Suitable for supply charged with an injectant.
8. Prevention of accidental needle retraction.
9. Low production costs.
10. Firm and compact for safe disposal.
The urgent need to address this problem has been widely acknowledged and many relevant designs tabled. One such design is known from U.S. Pat. No. 5,211,628 and this document discloses a hypodermic syringe including a housing, a plunger, a needle carrier with a needle mounted thereto positionable during assembly of the syringe within the housing with the needle extending therefrom, a sheath mounted to the needle carrier and surrounding the needle, and stored energy means, the plunger and stored energy means being configured such that when the syringe is used and the plunger reaches the needle carrier, it becomes attached thereto and the stored energy in the stored energy means is released to retract the needle carrier and the needle into the housing.
Similar hypodermic syringes having means for retracting the needle into the plunger after use are known from U.S. Pat. No. 5,324,265, EP-A-0505330 and WO91/03269A.
It is also known from U.S. Pat. No. 5,211,628 to provide a method of assembling a hypodermic syringe that includes the steps of mounting a needle to the needle carrier and positioning the needle carrier in the housing such that the needle extends therefrom.
A hypodermic syringe according to the present invention is provided with a housing configured such that a needle carrier with the needle mounted thereto is positionable during assembly of the syringe within the housing with the needle pre-sheathed.
In a preferred embodiment, the plunger comprises a casing and a piston located and movable within the casing.
Preferably, the stored energy means is a spring co-operating between the piston and the casing and has a releasable retaining means to release stored energy within the spring.
A method of assembling a hypodermic syringe according to the present invention includes the step of mounting a sheath to the needle carrier over the needle before positioning the needle carrier in the housing.